Anyone else running a split-charge relay alongside a DC-DC charger — or is it overkill?

So I've been scratching my head over this for a few weeks now. I've got a 2019 Transit Custom that I'm slowly converting, and I've already fitted a Renogy 40A DC-DC charger to properly charge my 200Ah lithium leisure battery from the alternator. Works a treat. But I've also got an old split-charge relay sitting in the garage from a previous van, and part of me wonders whether wiring it in as well would give me any benefit — or whether it'd just cause problems.

The way I understand it, the DC-DC charger already handles the voltage regulation side of things, so the relay would be a bit redundant. My concern is mainly about longer driving days — I'm doing a lot of touring up in Scotland this summer, sometimes 3–4 hours behind the wheel — and I want to make sure I'm pulling as much charge as possible without cooking the alternator on a hot day.

Has anyone actually run both together, or is the consensus that a decent DC-DC charger is all you need? I've seen a few builds on here where people have gone belt-and-braces with both, but I can't tell if there's a genuine reason or if it's just habit from the older lead-acid days. Would love to hear what setups people are actually running day-to-day.

MarineDoug | Posts: 847

@BrianKnight86 Totally understand the temptation to belt-and-braces it, but honestly the DC-DC charger is doing the heavy lifting already — it's managing the charge profile properly and protecting your starter battery from being drained. A split-charge relay alongside it is largely redundant and could actually cause headaches, particularly if the relay tries to parallel the banks at inopportune moments.

Where I have seen a relay make sense is as a pure backup/emergency path — wired through a manual switch only, never automatic. That way if your DC-DC packs up on a long trip you've got a fallback.

What's your leisure battery setup — AGM, lithium? That'll change the conversation somewhat. Lithium especially really does need the DC-DC doing its job properly rather than a relay chucking unregulated current at it.

ThistleWalker | Posts: 312

@BrianKnight86 Worth considering what you'd actually gain from adding the relay. Your DC-DC charger is already doing the clever work — regulating voltage properly for your leisure battery chemistry and protecting the alternator from being hammered. A split-charge relay on top doesn't add meaningful redundancy, it just introduces another potential failure point and could confuse your charging profile.

Where a relay genuinely earns its keep is if you're running something like a 12V compressor fridge directly off the vehicle side — keeping that isolated from your leisure bank without involving the DC-DC at all. But for charging purposes on a modern Transit with smart alternator? The Renogy alone is the right tool. Save the money and put it toward decent fusing instead.

BurnSophie | Posts: 203

Running a narrowboat setup taught me something relevant here — I went through exactly this logic spiral before settling on just the DC-DC (Victron Orion-Tr Smart 30A in my case).

The relay actually fights the DC-DC charger in certain conditions. Your Renogy is doing intelligent multi-stage charging; a relay upstream can cause voltage sensing confusion and odd cut-in/cut-out behaviour, particularly at low alternator speeds like tickover on a canal lock approach — or presumably a campsite queue.

What I'd genuinely consider instead: if you want redundancy, fit a second DC-DC unit wired in parallel. Cleaner, no conflicting logic, and you get double the charge current on long runs.

The belt-and-braces instinct is sound, but these two devices aren't really designed to coexist on the same circuit without careful isolation.

ThingamyBob62 | Posts: 156

Done exactly this in my Transit L2 — ran a VSR alongside my Victron Orion 30A for about six months thinking the same thing. Pulled the relay out in the end.

The DC-DC is doing the actual work, the relay just adds a potential fault point. Only time I'd keep both is if you've got a massive alternator and want raw bulk charge speed plus proper absorption stages, but even then it gets messy with the two systems arguing over voltage.

@BrianKnight86 what's your leisure bank — lithium or AGM? Makes a difference to the answer tbh. Lithium especially, you really don't want an unregulated VSR dumping current straight in.

BoatMartin | Posts: 847

@BrianKnight86 Coming from a liveaboard background like @BurnSophie, I'd say ditch the split-charge relay entirely. The whole point of a DC-DC charger is that it properly manages the charging profile — a VSR sitting alongside it just introduces potential for confusion, particularly if both try to respond to the same voltage conditions simultaneously.

One thing nobody's mentioned yet: with a 2019 Transit Custom you'll have a smart alternator with variable voltage output. A relay-based system will behave erratically with those because the voltage fluctuates deliberately. Your Renogy DC-DC handles that elegantly by design. Adding a relay back into the mix rather undermines what you've paid good money for.

Save yourself the wiring headache and trust the DC-DC to do its job properly. Simpler systems fail less often — that's been my experience across boats and vans alike.

DorsetCruiser | Posts: 412

@BrianKnight86 One thing nobody's mentioned yet — check what your Transit's smart alternator is actually doing voltage-wise. The 2019 Custom uses Euro 6 engine management, and those variable-voltage alternators can drop surprisingly low during certain load conditions. Your Renogy DC-DC will handle that fine, but a VSR alongside it might actually prevent the DC-DC from doing its job properly by partially bypassing it during those voltage dips. Had this exact issue in my Defender build before I properly understood what was happening. The DC-DC charger exists specifically because modern alternators behave oddly — adding a relay back into the mix can undermine the whole point. That said, if you're worried about a DC-DC failure leaving you stranded, a manual override switch between the batteries might give you peace of mind without the complications. Worth checking the Renogy specs on minimum input voltage too.